Add Library::new_with_flags

[tgross35]

Sometimes it would be convenient to keep the safer lifetime-bound API, but adjust the way the library gets loaded (e.g. to use eager binding). Add Library::new_with_flags and a LoadFlags type to provide an easier way to do this.

[nagisa]

Converting a os::Library into plain Library with an .into() seems pretty straightforward to me, so the safer API is within arm's reach at all times.

The fact that LoadFlags is a OS dependent typedef seems iffy -- code written in a portable seeming way isn't actually portable. More generally I consider that to be a major footgun with exposing any sort of exposition of OS-specific details outside the os::* modules and unless there is a very very good reason I would rather not break the os::* encapsulation. At least not until rust-lang/rust#41619 is implemented and libstd (and thus the ecosystem) expectations change with regards to what looks "portable".

[tgross35]

I don't know why I didn't consider just .intoing it. That's way easier, thanks!